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<a name="Values-From-Inferior-In-Guile"></a>
<div class="header">
<p>
Next: <a href="Arithmetic-In-Guile.html#Arithmetic-In-Guile" accesskey="n" rel="next">Arithmetic In Guile</a>, Previous: <a href="Guile-Exception-Handling.html#Guile-Exception-Handling" accesskey="p" rel="prev">Guile Exception Handling</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Values-From-Inferior-In-Guile-1"></a>
<h4 class="subsubsection">23.3.3.5 Values From Inferior In Guile</h4>
<a name="index-values-from-inferior_002c-in-guile"></a>
<a name="index-guile_002c-working-with-values-from-inferior"></a>

<a name="index-_003cgdb_003avalue_003e"></a>
<p><small>GDB</small> provides values it obtains from the inferior program in
an object of type <code>&lt;gdb:value&gt;</code>.  <small>GDB</small> uses this object
for its internal bookkeeping of the inferior&rsquo;s values, and for
fetching values when necessary.
</p>
<p><small>GDB</small> does not memoize <code>&lt;gdb:value&gt;</code> objects.
<code>make-value</code> always returns a fresh object.
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) guile (eq? (make-value 1) (make-value 1))
$1 = #f
(gdb) guile (equal? (make-value 1) (make-value 1))
$1 = #t
</pre></div>

<p>A <code>&lt;gdb:value&gt;</code> that represents a function can be executed via
inferior function call with <code>value-call</code>.
Any arguments provided to the call must match the function&rsquo;s prototype,
and must be provided in the order specified by that prototype.
</p>
<p>For example, <code>some-val</code> is a <code>&lt;gdb:value&gt;</code> instance
representing a function that takes two integers as arguments.  To
execute this function, call it like so:
</p>
<div class="smallexample">
<pre class="smallexample">(define result (value-call some-val 10 20))
</pre></div>

<p>Any values returned from a function call are <code>&lt;gdb:value&gt;</code> objects.
</p>
<p>Note: Unlike Python scripting in <small>GDB</small>,
inferior values that are simple scalars cannot be used directly in
Scheme expressions that are valid for the value&rsquo;s data type.
For example, <code>(+ (parse-and-eval &quot;int_variable&quot;) 2)</code> does not work.
And inferior values that are structures or instances of some class cannot
be accessed using any special syntax, instead <code>value-field</code> must be used.
</p>
<p>The following value-related procedures are provided by the
<code>(gdb)</code> module.
</p>
<dl>
<dt><a name="index-value_003f"></a>Scheme Procedure: <strong>value?</strong> <em>object</em></dt>
<dd><p>Return <code>#t</code> if <var>object</var> is a <code>&lt;gdb:value&gt;</code> object.
Otherwise return <code>#f</code>.
</p></dd></dl>

<dl>
<dt><a name="index-make_002dvalue"></a>Scheme Procedure: <strong>make-value</strong> <em>value <span class="roman">[</span>#:type type<span class="roman">]</span></em></dt>
<dd><p>Many Scheme values can be converted directly to a <code>&lt;gdb:value&gt;</code>
with this procedure.  If <var>type</var> is specified, the result is a value
of this type, and if <var>value</var> can&rsquo;t be represented with this type
an exception is thrown.  Otherwise the type of the result is determined from
<var>value</var> as described below.
</p>
<p>See <a href="Architectures-In-Guile.html#Architectures-In-Guile">Architectures In Guile</a>, for a list of the builtin
types for an architecture.
</p>
<p>Here&rsquo;s how Scheme values are converted when <var>type</var> argument to
<code>make-value</code> is not specified:
</p>
<dl compact="compact">
<dt>Scheme boolean</dt>
<dd><p>A Scheme boolean is converted the boolean type for the current language.
</p>
</dd>
<dt>Scheme integer</dt>
<dd><p>A Scheme integer is converted to the first of a C <code>int</code>,
<code>unsigned int</code>, <code>long</code>, <code>unsigned long</code>,
<code>long long</code> or <code>unsigned long long</code> type
for the current architecture that can represent the value.
</p>
<p>If the Scheme integer cannot be represented as a target integer
an <code>out-of-range</code> exception is thrown.
</p>
</dd>
<dt>Scheme real</dt>
<dd><p>A Scheme real is converted to the C <code>double</code> type for the
current architecture.
</p>
</dd>
<dt>Scheme string</dt>
<dd><p>A Scheme string is converted to a string in the current target
language using the current target encoding.
Characters that cannot be represented in the current target encoding
are replaced with the corresponding escape sequence.  This is Guile&rsquo;s
<code>SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE</code> conversion strategy
(see <a href="http://www.gnu.org/software/guile/manual/html_node/Strings.html#Strings">Strings</a> in <cite>GNU Guile Reference Manual</cite>).
</p>
<p>Passing <var>type</var> is not supported in this case,
if it is provided a <code>wrong-type-arg</code> exception is thrown.
</p>
</dd>
<dt><code>&lt;gdb:lazy-string&gt;</code></dt>
<dd><p>If <var>value</var> is a <code>&lt;gdb:lazy-string&gt;</code> object (see <a href="Lazy-Strings-In-Guile.html#Lazy-Strings-In-Guile">Lazy Strings In Guile</a>), then the <code>lazy-string-&gt;value</code> procedure is called, and
its result is used.
</p>
<p>Passing <var>type</var> is not supported in this case,
if it is provided a <code>wrong-type-arg</code> exception is thrown.
</p>
</dd>
<dt>Scheme bytevector</dt>
<dd><p>If <var>value</var> is a Scheme bytevector and <var>type</var> is provided,
<var>value</var> must be the same size, in bytes, of values of type <var>type</var>,
and the result is essentially created by using <code>memcpy</code>.
</p>
<p>If <var>value</var> is a Scheme bytevector and <var>type</var> is not provided,
the result is an array of type <code>uint8</code> of the same length.
</p></dd>
</dl>
</dd></dl>

<a name="index-optimized-out-value-in-guile"></a>
<dl>
<dt><a name="index-value_002doptimized_002dout_003f"></a>Scheme Procedure: <strong>value-optimized-out?</strong> <em>value</em></dt>
<dd><p>Return <code>#t</code> if the compiler optimized out <var>value</var>,
thus it is not available for fetching from the inferior.
Otherwise return <code>#f</code>.
</p></dd></dl>

<dl>
<dt><a name="index-value_002daddress"></a>Scheme Procedure: <strong>value-address</strong> <em>value</em></dt>
<dd><p>If <var>value</var> is addressable, returns a
<code>&lt;gdb:value&gt;</code> object representing the address.
Otherwise, <code>#f</code> is returned.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dtype"></a>Scheme Procedure: <strong>value-type</strong> <em>value</em></dt>
<dd><p>Return the type of <var>value</var> as a <code>&lt;gdb:type&gt;</code> object
(see <a href="Types-In-Guile.html#Types-In-Guile">Types In Guile</a>).
</p></dd></dl>

<dl>
<dt><a name="index-value_002ddynamic_002dtype"></a>Scheme Procedure: <strong>value-dynamic-type</strong> <em>value</em></dt>
<dd><p>Return the dynamic type of <var>value</var>.  This uses C<tt>++</tt> run-time
type information (<acronym>RTTI</acronym>) to determine the dynamic type of the
value.  If the value is of class type, it will return the class in
which the value is embedded, if any.  If the value is of pointer or
reference to a class type, it will compute the dynamic type of the
referenced object, and return a pointer or reference to that type,
respectively.  In all other cases, it will return the value&rsquo;s static
type.
</p>
<p>Note that this feature will only work when debugging a C<tt>++</tt> program
that includes <acronym>RTTI</acronym> for the object in question.  Otherwise,
it will just return the static type of the value as in <kbd>ptype foo</kbd>.
See <a href="Symbols.html#Symbols">ptype</a>.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dcast"></a>Scheme Procedure: <strong>value-cast</strong> <em>value type</em></dt>
<dd><p>Return a new instance of <code>&lt;gdb:value&gt;</code> that is the result of
casting <var>value</var> to the type described by <var>type</var>, which must
be a <code>&lt;gdb:type&gt;</code> object.  If the cast cannot be performed for some
reason, this method throws an exception.
</p></dd></dl>

<dl>
<dt><a name="index-value_002ddynamic_002dcast"></a>Scheme Procedure: <strong>value-dynamic-cast</strong> <em>value type</em></dt>
<dd><p>Like <code>value-cast</code>, but works as if the C<tt>++</tt> <code>dynamic_cast</code>
operator were used.  Consult a C<tt>++</tt> reference for details.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dreinterpret_002dcast"></a>Scheme Procedure: <strong>value-reinterpret-cast</strong> <em>value type</em></dt>
<dd><p>Like <code>value-cast</code>, but works as if the C<tt>++</tt> <code>reinterpret_cast</code>
operator were used.  Consult a C<tt>++</tt> reference for details.
</p></dd></dl>

<dl>
<dt><a name="index-value_002ddereference"></a>Scheme Procedure: <strong>value-dereference</strong> <em>value</em></dt>
<dd><p>For pointer data types, this method returns a new <code>&lt;gdb:value&gt;</code> object
whose contents is the object pointed to by <var>value</var>.  For example, if
<code>foo</code> is a C pointer to an <code>int</code>, declared in your C program as
</p>
<div class="smallexample">
<pre class="smallexample">int *foo;
</pre></div>

<p>then you can use the corresponding <code>&lt;gdb:value&gt;</code> to access what
<code>foo</code> points to like this:
</p>
<div class="smallexample">
<pre class="smallexample">(define bar (value-dereference foo))
</pre></div>

<p>The result <code>bar</code> will be a <code>&lt;gdb:value&gt;</code> object holding the
value pointed to by <code>foo</code>.
</p>
<p>A similar function <code>value-referenced-value</code> exists which also
returns <code>&lt;gdb:value&gt;</code> objects corresonding to the values pointed to
by pointer values (and additionally, values referenced by reference
values).  However, the behavior of <code>value-dereference</code>
differs from <code>value-referenced-value</code> by the fact that the
behavior of <code>value-dereference</code> is identical to applying the C
unary operator <code>*</code> on a given value.  For example, consider a
reference to a pointer <code>ptrref</code>, declared in your C<tt>++</tt> program
as
</p>
<div class="smallexample">
<pre class="smallexample">typedef int *intptr;
...
int val = 10;
intptr ptr = &amp;val;
intptr &amp;ptrref = ptr;
</pre></div>

<p>Though <code>ptrref</code> is a reference value, one can apply the method
<code>value-dereference</code> to the <code>&lt;gdb:value&gt;</code> object corresponding
to it and obtain a <code>&lt;gdb:value&gt;</code> which is identical to that
corresponding to <code>val</code>.  However, if you apply the method
<code>value-referenced-value</code>, the result would be a <code>&lt;gdb:value&gt;</code>
object identical to that corresponding to <code>ptr</code>.
</p>
<div class="smallexample">
<pre class="smallexample">(define scm-ptrref (parse-and-eval &quot;ptrref&quot;))
(define scm-val (value-dereference scm-ptrref))
(define scm-ptr (value-referenced-value scm-ptrref))
</pre></div>

<p>The <code>&lt;gdb:value&gt;</code> object <code>scm-val</code> is identical to that
corresponding to <code>val</code>, and <code>scm-ptr</code> is identical to that
corresponding to <code>ptr</code>.  In general, <code>value-dereference</code> can
be applied whenever the C unary operator <code>*</code> can be applied
to the corresponding C value.  For those cases where applying both
<code>value-dereference</code> and <code>value-referenced-value</code> is allowed,
the results obtained need not be identical (as we have seen in the above
example).  The results are however identical when applied on
<code>&lt;gdb:value&gt;</code> objects corresponding to pointers (<code>&lt;gdb:value&gt;</code>
objects with type code <code>TYPE_CODE_PTR</code>) in a C/C<tt>++</tt> program.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dreferenced_002dvalue"></a>Scheme Procedure: <strong>value-referenced-value</strong> <em>value</em></dt>
<dd><p>For pointer or reference data types, this method returns a new
<code>&lt;gdb:value&gt;</code> object corresponding to the value referenced by the
pointer/reference value.  For pointer data types,
<code>value-dereference</code> and <code>value-referenced-value</code> produce
identical results.  The difference between these methods is that
<code>value-dereference</code> cannot get the values referenced by reference
values.  For example, consider a reference to an <code>int</code>, declared
in your C<tt>++</tt> program as
</p>
<div class="smallexample">
<pre class="smallexample">int val = 10;
int &amp;ref = val;
</pre></div>

<p>then applying <code>value-dereference</code> to the <code>&lt;gdb:value&gt;</code> object
corresponding to <code>ref</code> will result in an error, while applying
<code>value-referenced-value</code> will result in a <code>&lt;gdb:value&gt;</code> object
identical to that corresponding to <code>val</code>.
</p>
<div class="smallexample">
<pre class="smallexample">(define scm-ref (parse-and-eval &quot;ref&quot;))
(define err-ref (value-dereference scm-ref))      ;; error
(define scm-val (value-referenced-value scm-ref)) ;; ok
</pre></div>

<p>The <code>&lt;gdb:value&gt;</code> object <code>scm-val</code> is identical to that
corresponding to <code>val</code>.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dfield"></a>Scheme Procedure: <strong>value-field</strong> <em>value field-name</em></dt>
<dd><p>Return field <var>field-name</var> from <code>&lt;gdb:value&gt;</code> object <var>value</var>.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dsubscript"></a>Scheme Procedure: <strong>value-subscript</strong> <em>value index</em></dt>
<dd><p>Return the value of array <var>value</var> at index <var>index</var>.
The <var>value</var> argument must be a subscriptable <code>&lt;gdb:value&gt;</code> object.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dcall"></a>Scheme Procedure: <strong>value-call</strong> <em>value arg-list</em></dt>
<dd><p>Perform an inferior function call, taking <var>value</var> as a pointer
to the function to call.
Each element of list <var>arg-list</var> must be a &lt;gdb:value&gt; object or an object
that can be converted to a value.
The result is the value returned by the function.
</p></dd></dl>

<dl>
<dt><a name="index-value_002d_003ebool"></a>Scheme Procedure: <strong>value-&gt;bool</strong> <em>value</em></dt>
<dd><p>Return the Scheme boolean representing <code>&lt;gdb:value&gt;</code> <var>value</var>.
The value must be &ldquo;integer like&rdquo;.  Pointers are ok.
</p></dd></dl>

<dl>
<dt><a name="index-value_002d_003einteger"></a>Scheme Procedure: <strong>value-&gt;integer</strong></dt>
<dd><p>Return the Scheme integer representing <code>&lt;gdb:value&gt;</code> <var>value</var>.
The value must be &ldquo;integer like&rdquo;.  Pointers are ok.
</p></dd></dl>

<dl>
<dt><a name="index-value_002d_003ereal"></a>Scheme Procedure: <strong>value-&gt;real</strong></dt>
<dd><p>Return the Scheme real number representing <code>&lt;gdb:value&gt;</code> <var>value</var>.
The value must be a number.
</p></dd></dl>

<dl>
<dt><a name="index-value_002d_003ebytevector"></a>Scheme Procedure: <strong>value-&gt;bytevector</strong></dt>
<dd><p>Return a Scheme bytevector with the raw contents of <code>&lt;gdb:value&gt;</code>
<var>value</var>.  No transformation, endian or otherwise, is performed.
</p></dd></dl>

<dl>
<dt><a name="index-value_002d_003estring"></a>Scheme Procedure: <strong>value-&gt;string</strong> <em>value <span class="roman">[</span>#:encoding encoding<span class="roman">]</span> <span class="roman">[</span>#:errors errors<span class="roman">]</span> <span class="roman">[</span>#:length length<span class="roman">]</span></em></dt>
<dd><p>If <var>value&gt;</var> represents a string, then this method
converts the contents to a Guile string.  Otherwise, this method will
throw an exception.
</p>
<p>Values are interpreted as strings according to the rules of the
current language.  If the optional length argument is given, the
string will be converted to that length, and will include any embedded
zeroes that the string may contain.  Otherwise, for languages
where the string is zero-terminated, the entire string will be
converted.
</p>
<p>For example, in C-like languages, a value is a string if it is a pointer
to or an array of characters or ints of type <code>wchar_t</code>, <code>char16_t</code>,
or <code>char32_t</code>.
</p>
<p>If the optional <var>encoding</var> argument is given, it must be a string
naming the encoding of the string in the <code>&lt;gdb:value&gt;</code>, such as
<code>&quot;ascii&quot;</code>, <code>&quot;iso-8859-6&quot;</code> or <code>&quot;utf-8&quot;</code>.  It accepts
the same encodings as the corresponding argument to Guile&rsquo;s
<code>scm_from_stringn</code> function, and the Guile codec machinery will be used
to convert the string.  If <var>encoding</var> is not given, or if
<var>encoding</var> is the empty string, then either the <code>target-charset</code>
(see <a href="Character-Sets.html#Character-Sets">Character Sets</a>) will be used, or a language-specific encoding
will be used, if the current language is able to supply one.
</p>
<p>The optional <var>errors</var> argument is one of <code>#f</code>, <code>error</code> or
<code>substitute</code>.  <code>error</code> and <code>substitute</code> must be symbols.
If <var>errors</var> is not specified, or if its value is <code>#f</code>, then the
default conversion strategy is used, which is set with the Scheme function
<code>set-port-conversion-strategy!</code>.
If the value is <code>'error</code> then an exception is thrown if there is any
conversion error.  If the value is <code>'substitute</code> then any conversion
error is replaced with question marks.
See <a href="http://www.gnu.org/software/guile/manual/html_node/Strings.html#Strings">Strings</a> in <cite>GNU Guile Reference Manual</cite>.
</p>
<p>If the optional <var>length</var> argument is given, the string will be
fetched and converted to the given length.
The length must be a Scheme integer and not a <code>&lt;gdb:value&gt;</code> integer.
</p></dd></dl>

<dl>
<dt><a name="index-value_002d_003elazy_002dstring"></a>Scheme Procedure: <strong>value-&gt;lazy-string</strong> <em>value <span class="roman">[</span>#:encoding encoding<span class="roman">]</span> <span class="roman">[</span>#:length length<span class="roman">]</span></em></dt>
<dd><p>If this <code>&lt;gdb:value&gt;</code> represents a string, then this method
converts <var>value</var> to a <code>&lt;gdb:lazy-string</code> (see <a href="Lazy-Strings-In-Guile.html#Lazy-Strings-In-Guile">Lazy Strings In Guile</a>).  Otherwise, this method will throw an exception.
</p>
<p>If the optional <var>encoding</var> argument is given, it must be a string
naming the encoding of the <code>&lt;gdb:lazy-string</code>.  Some examples are:
<code>&quot;ascii&quot;</code>, <code>&quot;iso-8859-6&quot;</code> or <code>&quot;utf-8&quot;</code>.  If the
<var>encoding</var> argument is an encoding that <small>GDB</small> does not
recognize, <small>GDB</small> will raise an error.
</p>
<p>When a lazy string is printed, the <small>GDB</small> encoding machinery is
used to convert the string during printing.  If the optional
<var>encoding</var> argument is not provided, or is an empty string,
<small>GDB</small> will automatically select the encoding most suitable for
the string type.  For further information on encoding in <small>GDB</small>
please see <a href="Character-Sets.html#Character-Sets">Character Sets</a>.
</p>
<p>If the optional <var>length</var> argument is given, the string will be
fetched and encoded to the length of characters specified.  If
the <var>length</var> argument is not provided, the string will be fetched
and encoded until a null of appropriate width is found.
The length must be a Scheme integer and not a <code>&lt;gdb:value&gt;</code> integer.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dlazy_003f"></a>Scheme Procedure: <strong>value-lazy?</strong> <em>value</em></dt>
<dd><p>Return <code>#t</code> if <var>value</var> has not yet been fetched
from the inferior.
Otherwise return <code>#f</code>.
<small>GDB</small> does not fetch values until necessary, for efficiency.
For example:
</p>
<div class="smallexample">
<pre class="smallexample">(define myval (parse-and-eval &quot;somevar&quot;))
</pre></div>

<p>The value of <code>somevar</code> is not fetched at this time.  It will be
fetched when the value is needed, or when the <code>fetch-lazy</code>
procedure is invoked.
</p></dd></dl>

<dl>
<dt><a name="index-make_002dlazy_002dvalue"></a>Scheme Procedure: <strong>make-lazy-value</strong> <em>type address</em></dt>
<dd><p>Return a <code>&lt;gdb:value&gt;</code> that will be lazily fetched from the
target.  The object of type <code>&lt;gdb:type&gt;</code> whose value to fetch is
specified by its <var>type</var> and its target memory <var>address</var>, which
is a Scheme integer.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dfetch_002dlazy_0021"></a>Scheme Procedure: <strong>value-fetch-lazy!</strong> <em>value</em></dt>
<dd><p>If <var>value</var> is a lazy value (<code>(value-lazy? value)</code> is <code>#t</code>),
then the value is fetched from the inferior.
Any errors that occur in the process will produce a Guile exception.
</p>
<p>If <var>value</var> is not a lazy value, this method has no effect.
</p>
<p>The result of this function is unspecified.
</p></dd></dl>

<dl>
<dt><a name="index-value_002dprint"></a>Scheme Procedure: <strong>value-print</strong> <em>value</em></dt>
<dd><p>Return the string representation (print form) of <code>&lt;gdb:value&gt;</code>
<var>value</var>.
</p></dd></dl>

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